An image forming apparatus typically has both a normal mode and a power saving mode. In the normal mode, for example, an image fixing device is maintained at an appropriate temperature to fix a developer such as toner, and therefore the image forming apparatus is allowed to form an image as soon as a request for forming an image is received. However, power consumption is increased so as to constantly maintain the image fixing device at the appropriate temperature in the normal mode.
In a power saving mode, for example, the image fixing device as described above is maintained at a temperature higher than a room temperature but lower than the temperature in the normal mode. Therefore additional time is necessary for the image forming apparatus to heat the image fixing device so as to reach the appropriate temperature after a request for forming an image is received. However, the power consumed while in the power saving mode may be reduced as compared to power consumed while in the normal mode.
The image forming apparatus as described above often has a plurality of CPUs including, but not limited to, for example, a main CPU and a sub CPU. In this case, in the normal mode, the main CPU controls an operation performed by the image forming apparatus. Specifically, the main CPU controls the image forming operation entirely. On the other hand, in the power saving mode, the sub CPU controls an operation performed by the image forming apparatus. Specifically, the sub CPU controls time for the image forming apparatus to operate in the power saving mode. In the power saving mode, the main CPU suspends its operation so as to reduce power consumption.
A technique that uses measured times to restore a facsimile apparatus to the normal mode from a power saving mode has been suggested. In this technique, the sub CPU has two types of timers, and causes two types of timers to each measure time differently from each other respectively.
This technique employs two types of timers which operate in accordance with an operation performed by a clock generator. A crystal oscillator is generally accurate but expensive when used as a clock generator. When such a crystal oscillator is used also for a sub CPU, the costs of apparatuses such as facsimiles, printers and copiers increase. On the other hand, when a less expensive but less accurate clock generator is used for a time measuring apparatus or the like, it may be difficult or insufficient to appropriately control the apparatus.